Castle Bleck is the setting of Chapter 8 in Super Paper Mario, and it is found after entering the Black Door on Flopside Tower. Located in a realm of darkness, it is Count Bleck's home. The castle is separated into four main areas, one for each Chapter section respectively: the Entry, the Foyer, the Interior, and the Inner Sanctum. Castle Bleck is one of the only two worlds (the other being The Bitlands) to have at least one boss battle in each area (O'Chunks in 8-1, Mimi in 8-2. Dimentio in 8-3, and Count Bleck and Super Dimentio in 8-4). Castle Bleck's theme was a remix of the Pure Hearts', Count Bleck's, and the Chaos Heart's theme, mixed with some 8-bit sound effects. According to The InterNed, the music that plays while Peach or Luigi are trying to escape Castle Bleck is titled "In the Darkness".
After Mario collects the Pure Heart in the Yold Ruins, Princess Peach finds herself lost within the castle. With the help of Private Koopa, she attempts to find a way to escape. However, when she finds a dead end, Nastasia catches up to her, and is about to place her under hypnosis when Dimentio intervenes, unseen, and teleports Peach out of the castle.
Later, after the Pure Heart in Merlee's Basement is found, Luigi also finds himself within Castle Bleck. With the help of a pair of Goombas, he attempts an escape just like Peach. Once again, a dead end is found (in actuality, this was the castle exit, but the door was in 3-D and could not be seen or used), and Luigi is taken captive and turned into Mr. L by Nastasia's hypnotic powers.Flopside. In Chapter 8-1, Bowser and O'Chunks face off against each other. When Bowser wins, the ceiling begins to fall; however, O'Chunks holds it up because of his honor code. Bowser stays behind, even as the two keep telling one another to leave. The ceiling apparently crushes them both. In 8-2, Mimi impersonates Merlon, and tells Mario to hit a nearby ! Block. Even though the trap is obvious, the player must fall into it to continue. When they get out of the pit that they've fallen into, Mimi impersonates Merlee for the third time in the game. She then asks the player a series of questions about their feared enemies (reminiscent of the questions Bowser asked Peach in the original Paper Mario). After going through the hallways filled with the enemies or items chosen as the responses, Mimi taunts Peach, who demands a one-on-one fight with her. After Peach wins, the floor begins to collapse. Peach manages to hold onto the floor, and even prevents Mimi from falling to her doom. She loses her grip, however, and they fall down.
In 8-3, the player encounters several rooms with mirrors as well as countless Dimentio clones. When they finally reach the end, Mario and Luigi end up chasing after Dimentio through the previous seven worlds. Once they catch up with him, Dimentio reveals his agenda against the Count and suggests that they team up so that all of them could control the Chaos Heart. Mario and Luigi, however, refuse. Dimentio then plans to attack both of them, but, when he insults Luigi's mustache, Luigi decides to defeat him on his own. When Dimentio is defeated, however, he presumably ends the game of both himself and Luigi.
Mario, now on his own, goes through the maze of the final area of Castle Bleck. He eventually reaches Count Bleck's chambers. After Tippi and Bleck argue, Bleck sends Nastasia away and begins to fight Mario. At first, Mario's attacks have no effect; however, it is then revealed that Bowser, Peach, and Luigi all survived, as they enter the chamber. The Pure Hearts disable Count Bleck's barrier, and, together, the four heroes defeat him.
Nastasia reappears, and Count Bleck tells the heroes to finish him off so that The Void will disappear. However, Tippi is reluctant to do so. They had only just been reunited, and she didn't want them to be ripped apart so soon. At that moment, a supposedly fatal blast of energy is sent toward Count Bleck, but Nastasia takes the blow instead. Dimentio appears, saying that the Chaos Heart would not disappear if he continued to control it. It is revealed that, after Luigi had defeated him, Dimentio had planted a mind-control seed in his subconscious, and it sprouts into a Floro Sprout. Dimentio then sends Tippi and Count Bleck to Dimension D, and continues with his plans. The Chaos Heart, Dimentio, and Luigi merge, creating a freakish creature reminiscent of both Brobot and Dimentio, called Super Dimentio. Mario, Bowser, and Peach are currently unable to damage it.
Meanwhile in Dimension D, Tippi argues with Bleck that they have to do something to help Mario. Bleck, however, says that it's a pointless endeavor. They exchange a few more words before Mimi and O'Chunks appear, lending the count their support. The Pure Hearts, thought to have been wasted on Bleck, reappear, and Tippi takes them to Mario. Super Dimentio is able to be damaged now, and the final showdown commences.
Mario, Peach, and Bowser prevail, and Super Dimentio is defeated. However, in addition to Luigi's normal self, he leaves behind one fragment of his power, so that the Void would still destroy all worlds even in his absence.
There was only one way to stop it: giving the Pure Hearts enough power through an act of true love. However, when that happened, everyone who would take part in it will no longer exist. Everyone goes through the final door, and they end up at the altar where Peach and Bowser were forced into marriage at the start of the game. Bleck and Tippi go up to it, and proclaim their love for each other. In their supposed final moments, they are married, and the Chaos Heart is stopped.
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